A lithographic apparatus is a machine that applies a desired pattern onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of logic and/or memory chips, termed integrated circuits (ICs) herein. In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. These target portions are commonly referred to as “fields”.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. This may be termed metrology. Various tools for making such measurements are known, including scanning electron microscopes (SEMs), which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of optical tools or scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a diffraction “spectrum” from which a property of interest of the target can be determined.
At the same time, known inspection techniques employ radiation in the visible or ultraviolet waveband (e.g. greater than 200 nm). This limits the smallest features that can be measured, so that the technique can no longer measure directly the smallest features made in modern lithographic processes. To allow measurement of smaller structures, it has been proposed to use radiation of shorter wavelengths similar, for example, to the extreme ultraviolet (EUV) wavelengths used in EUV lithography. Such wavelengths may be in the range 1 to 100 nm, for example, or 1 to 125 nm. Part or all of this wavelength range may also be referred to as soft x-ray (SXR) wavelengths. Some authors may use SXR to refer to a narrower range of wavelengths, for example in the range 1-10 nm or 1-20 nm. For the purposes of the methods and apparatus disclosed herein, these terms SXR and EUV will be used without implying any hard distinction. Metrology using harder x-rays, for example in the range 0.1-1 nm is also contemplated.
Convenient sources of SXR radiation include HHG sources, in which infrared pump radiation from a laser is converted to shorter wavelength radiation by interaction with a gaseous medium. HHG sources are available for example from KMLabs, Boulder Colo., USA (http://www.kmlabs.com/).
Since the SXR photons of interest have a very short penetration depth in any medium, the gaseous medium may take the form of a gas jet located in a low-pressure (near vacuum) environment. The gas jet may be freely ejected from a nozzle, or confined within a waveguide structure that prolongs its interaction with the pump radiation.
Currently available SXR sources are very limited in output power. To enable inspection tools that both have high resolution and high throughput, there is therefore a need for high power SXR sources.
FIG. 1 shows a block schematic sketch of an apparatus 100 for use as an HHG source 100. The apparatus 100 comprises a pulsed high power infrared or optical laser 102, a chamber 104 comprising a radiation input 106 and a radiation output 108, and a vacuum optical system 110. The laser 102 emits driving radiation, which enters the chamber 104 through the radiation input 106 and is incident on a gas target 112 located at an interaction region 114 within the chamber 104. The gas target 112 comprises a small volume (typically several cubic mm) of a particular gas (e.g., a noble gas, nitrogen, oxygen or carbon dioxide). Other media, such as metallic plasmas (e.g. aluminium plasma) may be used.
Due to interaction of the driving radiation emitted by the laser 102 with the gas atoms of the gas target 112, the gas target 112 will convert part of the driving radiation into emitted radiation, which in this case comprises radiation at a plurality of wavelengths in the range from 1 nm to 100 nm (termed SXR herein). The emitted radiation is emitted in a direction collinear with the incident driving radiation.
The SXR beam passes through the radiation output 108 and is subsequently manipulated and directed to a wafer to be inspected by the vacuum optical system 110.
Because air (and in fact any gas) heavily absorbs SXR radiation, the volume between the gas target and the wafer to be inspected is evacuated or nearly evacuated. The driving radiation is directed into the chamber 104 through the radiation input 106, which is a viewport typically made of fused silica or a comparable material. Since the driving radiation and the emitted radiation (SXR beam) are collinear, the driving radiation typically needs to be blocked to prevent it passing through the radiation output 108 and entering the vacuum optical system 110. This is typically done by incorporating a filter into the radiation output 108, which is placed in the emitted beam path and that is opaque to the driving radiation (e.g. opaque to infrared or visible light) but at least partially transparent to the emitted radiation beam. The filter may be manufactured using zirconium.
In known HHG sources, a significant proportion of the emitted radiation beam is absorbed by the laser blocking filter used at the radiation output 108 to block the driving radiation. This leads to a loss of emitted radiation output power of typically 50%.
In addition, the viewport at the radiation input 106 through which the driving radiation enters the chamber 104 of the apparatus 100 has a number of disadvantages.
For example, part of the driving radiation is reflected and/or absorbed by the viewport. This may lead to a transmission loss of about 5-10% of the incident driving radiation. Given that the emitted radiation intensity of an HHG source is determined at least in part by the power of the driving radiation reaching the gas target 112, mitigation of this transmission loss would directly lead to 5-10% higher emitted radiation intensity.
Also, to reach high emitted radiation intensities, there is a tendency to tightly focus the driving radiation onto the gas target 112. In such tight-focus configurations, the gas target 112 will typically be placed relatively close to the viewport. Therefore the driving radiation will already be partially focused to relatively small beam cross-section when it enters the viewport, leading to high heat load on the viewport surface. Therefore the accessible range of tight-focusing configurations in an HHG source is currently limited by the material properties of the viewport and cooling system capacity. Mitigation of this heat load problem would extend the accessible range of focusing geometries.
In addition, driving radiation propagating through the viewport is prone to beam degradation due to material defects and surface imperfections of the viewport. Beam degradation leads to reduced ability to focus of the driving radiation beam, which reduces the range of intensities that can be reached in the focal spot at the gas target 112. Since the HHG mechanism and thereby the properties of the emitted radiation beam sensitively depend on the driving radiation intensity distribution, beam degradation due to the viewport results in reduced control of the emitted radiation intensity and beam properties.